Heat shrinkable C2C4C6 terpolymer film

ABSTRACT

Biaxially stretched, heat shrinkable monolayer and multilayer films comprising very low density polyethylene terpolymers of monomers (a), (b) and (c), where (a) comprises ethylene, (b) comprises a C6-C8 alpha-olefin and (c) comprises 1-butene or 1-hexene, have a very good combination of physical properties and processability including high shrinkage values and puncture resistance.

This application is a Continuation of prior U.S. application Ser. No.07/892,637 filed Jun. 2, 1992 abandoned, which is a CONTINUATION ofapplication Ser. No. 07/286,019 filed Dec. 19, 1988, which applicationsare hereby incorporated by reference in their entireties, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to packaging films. In particular, thepresent invention relates to biaxially stretched, heat shrinkable filmsmade of copolymers of polyethylene.

Polyethylene is the name for a polymer whose basic structure ischaracterized by the chain CH₂CH₂_(n). Polyethylene homopolymer isgenerally described as being a solid which has a partially amorphousphase and partially crystalline phase with a density of between 0.915 to0.970 g/cm³. The relative crystallinity of polyethylene is known toaffect its physical properties. The amorphous phase imparts flexibilityand high impact strength while the crystalline phase imparts a highsoftening temperature and rigidity.

Unsubstituted polyethylene is generally referred to as high densityhomopolymer and has a crystallinity of 70 to 90 percent with a densitybetween about 0.96 to 0.97 g/cm³. Most commercially utilizedpolyethylenes are not unsubstituted homopolymer but instead have C₂-C₈alkyl groups attached to the basic chain. These substitutedpolyethylenes are also known as branched chain polyethylenes. Also,commercially available polyethylenes frequently include othersubstituent groups produced by copolymerization. Branching with alkylgroups generally reduces crystallinity, density and melting point. Thedensity of polyethylene is recognized as being closely connected to thecrystallinity. The physical properties of commercially availablepolyethylenes are also affected by average molecular weight andmolecular weight distribution, branching length and type ofsubstituents.

People skilled in the art generally refer to several broad categories ofpolymers and copolymers as “polyethylene.” Placement of a particularpolymer into one of these categories of “polyethylene” is frequentlybased upon the density of the “polyethylene” and often by additionalreference to the process by which it was made since the process oftendetermines the degree of branching, crystallinity and density. Ingeneral, the nomenclature used is nonspecific to a compound but refersinstead to a range of compositions. This range often includes bothhomopolymers and copolymers.

For example, “high density” polyethylene (HDPE) is ordinarily used inthe art to refer to both (a) homopolymers of densities between about0.960 to 0.970 g/cm³ and (b) copolymers of ethylene and an alpha-olefin(usually 1-butene or 1-hexene) which have densities between 0.940 and0.958 g/cm³. HDPE includes polymers made with Ziegler or Phillips typecatalysts and is also said to include high molecular weight“polyethylenes.” In contrast to HDPE, whose polymer chain has somebranching, are “ultra high molecular weight polyethylenes” which areessentially unbranched specialty polymers having a much higher molecularweight than the high molecular weight HDPE.

Hereinafter, the term “polyethylene” will be used (unless indicatedotherwise) to refer to ethylene homopolymers as well as copolymers ofethylene with alpha-olefins and the term will be used without regard tothe presence or absence of substituent branch groups.

Another broad grouping of polyethylene is “high pressure, low densitypolyethylene” (LDPE). The polyethylene industry began in the 1930's as aresult of the discovery of a commercial process for producing LDPE byImperial Chemical Industries, Ltd. researchers. LDPE is used todenominate branched homopolymers having densities between 0.915 and0.930 g/cm³ as well as copolymers containing polar groups resulting fromcopolymerization e.g. with vinyl acetate or ethyl acrylate. LDPEstypically contain long branches off the main chain (often termed“backbone”) with alkyl substituents of 2 to 8 carbon atoms.

In the 1970's a new grouping of polyethylene was commercialized—LinearLow Density Polyethylene (LLDPE). Only copolymers of ethylene withalpha-olefins are in this group, LLDPEs are presently recognized bythose skilled in the art as having densities from 0.915 to 0.940 g/cm³.The alpha-olefin utilized is usually 1-butene, 1-hexene, or 1-octene andZiegler-type catalysts are usually employed (although Phillips catalystsare also used to produce LLDPE having densities at the higher end of therange).

In the 1980's yet another grouping of polyethylene has come intoprominence—Very Low Density Polyethylene (VLDPE) which is also called“Ultra Low Density Polyethylene” (ULDPE). This grouping like LLDPEscomprise only copolymers of ethylene with alpha-olefins, usually1-butene, 1-hexene or 1-octene and are recognized by those skilled inthe art as having a high degree of linearity of structure with shortbranching rather than the long side branches characteristic of LDPE.However, VLDPEs have lower densities than LLDPEs. The densities ofVLDPEs are recognized by those skilled in the art to range between 0.860and 0.915 g/cm³. A process for making VLDPEs is described in EuropeanPatent Document publication number 120,503 whose text and drawing arehereby incorporated by reference into the present document.

Various types of polyethylene resins have long been used to producefilms having different properties. These polyethylenes have been usedalone, in blends and with copolymers in both monolayer and multilayerfilms for packaging applications for such food products as poultry,fresh red meat and processed meat. In the food industry greater use ofcentralized processing of foods in conjunction with increased handlingand long distance transportation have increased the demand for packagingfilms having superior properties.

In the poultry and meat segments of the food industry thermoplastic heatshrinkable flexible films are utilized to maintain freshness. Meat isfrequently sold fresh, frozen or cooked; therefore films advantageouslyprovide protection at various temperatures. Food items such as primaland subprimal cuts of beef, ground beef and processed meats are known touse coextruded, extrusion coated or laminated films which utilize suchcompositions as LLDPE, nylon, polyester, copolymer of vinylidenechloride (PVDC), ethylene-vinyl acetate copolymer (EVA) and ionomers.

It is generally known that selection of films for packaging foodproducts includes consideration of one or more criteria such as punctureresistance, shrinkability, shrink force, cost, sealability, stiffness,strength, printability, durability, barrier properties, machinability,optical properties such as haze and gloss, flex-crack resistance andgovernment approval for contact with food.

For example, several film materials containing polyethylene have beeneither used or proposed for packaging frozen poultry. In general,commercial poultry packaging operations require bags made from materialsable to withstand the following typical process and transfer steps:

1. Inserting a bird into a bag fabricated from a shrinkable film;

2. Evacuating the bag;

3. Clamping or otherwise sealing the neck of the bag;

4. Transporting the bird (e.g. by a conveyor belt) to a shrink tunnel;

5. Shrinking the bag tightly around the bird by exposing the bag to atemperature of about 90-95° C. for up to about six to eight seconds;

6. Quick freezing and storage of the packaged bird at temperatures aslow as −40° C.; and

7. Transporting the packaged bird from the commercial packer to theultimate user.

A film useful for frozen poultry packaging will include among itsdesirable properties the following:

a) A shrinkage value that yields a reduction in the area of the film ata temperature from 90-95° C. that is sufficient to conform the film tothe irregular shape of the bird;

b) a shrink force at a temperature of 90-95° C. is required that issufficient to pull the wings of the bird in tightly toward the body withsufficient residual shrink force to maintain a tight wrap around thebird; and

c) a puncture resistance sufficient to withstand the packaging operationitself as well as subsequent transport of the packaged bird.

All the above properties should be provided in a film at a minimum ofcost.

Several polyolefin films have previously been proposed for use aspoultry bags.

U.S. Pat. No. 3,555,604 (Pahlke) discloses that low density polyethylenemay be biaxially oriented to produce a film which is useful forpackaging foodstuffs such as turkey.

Multilayer biaxially oriented films have been proposed for poultry bagssuch as those described in U.S. Pat. No. 3,900,635 (Funderburk, Jr. etal) wherein a first layer comprises an ethylene homopolymer or copolymerand a second layer comprises a blend of an ionomer and a second ethylenehomopolymer or copolymer.

Also, various blends of different polyethylene resins have beenreported. For example, blends of LLDPE with LLDPE or LDPE have beenreported in the article by Utracki et al, “Linear Low DensityPolyethylene and Their Blends: Part 4 Shear Flow of LLDPE Blends withLLDPE and LDPE”, Polymer Engineering and Science, Vol. 27, No. 20, pp1512-1522 (mid-November, 1987). In its introduction, the above articlestates that . . . “at least 60% of LLDPE is sold in blends withpolyolefins or EVA (ethylene-vinyl acetate copolymers) (cite omitted).Amelioration of properties (e.g., puncture resistance), lowering ofmaterial cost or improvement of processability are the main reasons”.The article goes on to discuss data relating to blends of a LLDPE madefrom a copolymer of polyethylene with 1-butene with (a) a LLDPE madefrom a copolymer of polyethylene and 1-hexene, and (b) a LDPE.

Various VLDPEs have been suggested for use as suitable resins for makinga shrinkable multilayer or single layer film for food packaging.

U.S. Pat. No. 4,640,856 (Ferguson et al) discloses heat shrinkablemultilayer films containing VLDPE which are useful in packaging meat,poultry and dairy products. Ferguson, et al in describing theirthermoplastic polymeric layer also state that “in certain applicationsblends of VLDPE, LLDPE and/or EVA may be used to achieve desiredproperties”.

Other patents have disclosed use of VLDPE resins in film including U.S.Pat. Nos. 4,671,987; 4,720,427; and 4,726,997.

Various ethylene based terpolymer resins having densities below 0.915have been previously described. For example, EP Patent ApplicationPublication No. 144 716 (Carrick et al) discloses a process where“ethylene is copolymerized with one or more comonomers which comprise1-olefins having between 3 and 8 carbon atoms in their main carbonchains. The 1-olefin comonomers may be substituted or unsubstituted.Olefins such as propylene, 1-butene, 1-hexene, 1-octene and substitutedcomonomers such as 4-methyl-1-pentene-1 are preferred.” Copolymers aresaid to be formed having “densities generally in the range of less thanabout 0.87 g/cc to about 0.94 g/cc.” Although broadly suggesting aprocess for copolymerizing ethylene with one or more monomers, Carricket al does not have any specific examples of copolymers made with morethan two monomer components. Also, Carrick et al is silent regarding anyutility of the materials disclosed therein for making heat shrinkablefilms, for example, for packaging.

The concept of using copolymer resins having more than two monomers toform heat sealable films has been broadly disclosed, for example, inEuropean Patent Application Publication number 247,897 (Bossaert et al).Bossaert et al disclose films which are preferably based on propylenewhich are heat shrinkable, and may be biaxially oriented. These filmsare described as being useful for packaging. Bossaert et al are silentregarding any puncture or shrinkage properties of their film and do nothave any specific examples of copolymers made with more than two monomercomponents.

Heat shrinkable films comprising propylene-ethylene-alpha-olefinterpolymer are also known as shown by Japanese Patent ApplicationPublication Number 45306/1988 (Isaka et al). Isaka et al disclose apropylene-ethylene-alpha-olefin terpolymer heat-shrinkable film. Thisterpolymer film is described as containing less than ten weight percentethylene.

Also, various government regulatory approvals for various terpolymersfor use in contact with food have been or are being sought for suchterpolymers as ethylene-octene-butene terpolymer, ethylene-octene-hexeneterpolymer (See e.g. Fed. Reg. 23798 Jun. 24, 1988) orethylene-hexene-butene terpolymer (See 21 CFR 177.1520).

None of the foregoing publications have disclosed biaxially stretched,heat shrinkable films made from a very low density polyethyleneterpolymer of ethylene, 1-butene, and a C₆-C₈ alpha-olefin, or ethylene,1-hexene and either a C₆-C₈ alpha olefin. Also, presently known filmsused as poultry bags continue to suffer from insufficient punctureresistance, and/or shrinkability.

Puncture resistance is a useful property of packaging films in generaland an important property of food packaging films. Puncture resistanceis very important for films used in forming bags for poultry. Thesepoultry bags must have a high puncture resistance in order to withstandpackaging operations and transport as well as retail customer inspectionand handling. Punctured poultry bags not only expose the contained birdsto spoilage agents, but also allows leakage of liquid from within thebag. This leakage is highly undesirable to grocery shoppers andretailers. In retail poultry displays, leaked liquid often istransferred to adjacent products making displays and selection messy. Ashopper who places a punctured bag into a grocery cart may causemoisture damage to paper products or packaging. In addition, concernabout possible salmonella or other bacterial contamination via contactwith leaked poultry liquid increases the desirability of punctureresistant poultry packaging.

Punctured and leaking bags are still very much a problem in poultrypackaging. Recently, very low density polyethylene (VLDPE) resins havebeen utilized in making shrinkable packaging films including films forfood contact packaging.

One type of commercially available VLDPE is a copolymer of ethylene and1-butene sold by Union Carbide Corporation under the brand designationDFDA 1137, Natural 7. Disadvantageously, this resin has been found tohave low puncture resistance in packaging operations. In particular,where packaging films are exposed to elevated temperatures in a filmshrinking step of a packaging operation, puncture resistance isundesirably low.

Another type commercially available VLDPE is a copolymer of ethylene and1-octene sold by Dow Chemical Company under the brand designation Attane4001. While this film has improved puncture properties relative to DFDA1137, it has undesirably low shrinkage values.

An experimental VLDPE that is a copolymer of ethylene and 1-hexene wasobtained from Union Carbide Corporation under the experimental branddesignation DEFD 1569. In one experiment disclosed in the presentapplication, heat shrinkable, biaxially oriented films were made undersimilar conditions. A film made from this experimental ethylene,1-hexene VLDPE when compared to a film made from DFDA 1137, had asimilar dynamic puncture resistance, greater hot water punctureresistance and undesirably low shrinkage values.

Advantageously, a biaxially stretched, heat shrinkable film of thepresent invention may have both high dynamic puncture resistancerelative to similarly formed films made from commercially available1-butene based VLDPE resins and experimental 1-hexene based VLDPE resinsas well as high shrinkage values relative to similarly formed films madefrom commercially available 1-octene based VLDPE resins and experimental1-hexene based VLDPE resins. An inventive film has also been found tohave a high puncture resistance at elevated temperatures (hot waterpuncture resistance) relative to a similarly formed 1-butene based VLDPEfilm.

Although the broad concept of making ethylene-alpha olefin terpolymerresins has been previously disclosed in the art, heat shrinkable,biaxially stretched films of the specific terpolymers according to thepresent invention have not been taught in the prior art. Previously,where specific terpolymers have been disclosed in any detail, most oftenpropylene and/or a diene has been one of the terpolymer comonomers.These known terpolymer resins are generally synthetic elastomers havingproperties similar to rubber, which makes these materials generallyundesirable for use as the principal component of the packaging film.These elastomeric resins typically have very low crystallinity to thepoint of being amorphous with no definite crystalline melting pointunlike the resins utilized in the present invention. Moreover, theutility and properties of biaxially stretched, heat shrinkable, flexiblefilms comprising the specific ethylene, C₆-C₈ alpha-olefin, and 1-buteneor 1-hexene terpolymers according to the present invention have not beenpreviously disclosed. These previously unknown, useful and surprisingproperties of these novel films are now disclosed below for the firsttime in the present specification.

SUMMARY OF THE INVENTION

According to the present invention a novel biaxially stretched, heatshrinkable film comprising a terpolymer of monomers (a), (b) and (c),wherein monomer (a) comprises ethylene, monomer (b) comprises a C₆-C₈alpha-olefin such as 4-methyl-1-pentene, 1-hexene or 1-octene andmonomer (c) comprises 1-butene or 1-hexene, having a terpolymer densityless than 0.915 g/cm³ is provided. Advantageously, certain propertiesand combinations of properties of the biaxially stretched film of theinvention are superior to films made of copolymers of ethylene and theother terpolymer monomer components alone e.g. either 1-butene or1-hexene or the C₆-C₈ alpha-olefin utilized. In particular, theinventive heat shrinkable terpolymer films exhibit a desirablecombination of high shrinkage values and high dynamic and hot waterpuncture resistance which are advantageous for producing packaging bagse.g. for poultry, fresh red meat and processed foods such as processedmeat and cheese.

DESCRIPTION OF THE INVENTION

Very Low Density Polyethylenes (VLDPEs) are copolymers of ethylene andone or more alpha-olefins (such as propylene, 1-butene, 1-hexene or1-octene) which have densities between 0.915 and 0.860 g/cm³. Theterpolymers of the present invention are VLDPEs which may be made bysolution processes or fluidized bed processes. European PatentApplication 84 103441.6 having publication number 120503 (which ishereby incorporated by reference in its entirety into the presentapplication insofar as its teachings are consistent with the presentdisclosure) describes a suitable method for preparation of low density,low modulus ethylene copolymers utilizing a fluidized bed. Theseethylene copolymers are described as having a density of less than 0.915g/cm³ and a 1% secant modulus of less than 140,000 kPa and this processand resulting terpolymer resins which may be made by this process arebelieved to be suitable for the films of the present invention. The wellknown fluidized bed process such as the Unipol (Trademark of UnionCarbide Corporation) process and reactor may without undueexperimentation be adapted to produce suitable terpolymers of thepresent invention.

Catalyst selection is recognized by those of ordinary skill in the artto be an important variable parameter for modifying terpolymerpolymerization and resultant properties. Various catalysts are known inthe art as useful for modifying VLDPE polymer formulation. Examples ofvarious catalysts known to be useful in making very low densitypolyethylene, include titanium, magnesium or vanadium containingcompositions which are known in the art of polyethylene resinmanufacture. Suitable catalysts include those disclosed in EuropeanPatent Publication No. 120,503 and U.S. Pat. No. 4,508,842. It isbelieved that catalyst selection as well as other variables may bechanged or modified by those of ordinary skill in the art to arrive atsuitable and preferred terpolymer resins useful in the present inventionwithout undue experimentation.

Various VLDPEs are manufactured by and also available on either acommercial or experimental basis from Dow Chemical Company of Midland,Mich. U.S.A. and Union Carbide Corporation of Danbury, Conn., U.S.A.

Suitable VLDPE terpolymers useful in the present invention are made fromcopolymerization of ethylene with either 1-butene or 1-hexene, and atleast one C₆-C₈ alpha-olefin. Suitable C₆-C₈alpha-olefins include:4-methyl-1-pentene; 1-hexene, and 1-octene. A preferred terpolymercomprises a VLDPE copolymer of ethylene, 1-butene and 1-hexene(hereinafter termed C₂C₄C₆ VLDPE terpolymer).

VLDPE terpolymers of ethylene with either 1-butene or 1-hexene and aC₆-C₈ alpha-olefin according to the present invention have a density ofless than about 0.915 g/cm³ as measured by ASTM Standard Test Method D1505. Suitable terpolymers include those having a density between about0.915 and 0.860 gm/cm³ with those having a density ranging from 0.901 to0.905 g/cm³ being preferred and those having a density of about 0.905g/cm³ being especially preferred. Advantageously, the melt index (asmeasured by ASTM D-1238, Condition E) of the above terpolymers will beless than 2.0 dg/min with a melt index of 0.1 to 1.0 dg/min preferred,and a melt index of 0.1 to 0.3 dg/min. especially preferred as measuredby ASTM Test Method 1238. It is believed that biaxially stretched filmproperties improve with decreasing melt index.

Suitable terpolymer resins for making biaxially stretched films of thepresent invention may have a melt flow ratio (Ratio of: Flow Index asmeasured by ASTM D-1238, Condition F to Melt Index as measured by ASTMD-1238, Condition E) ranging from below 35 to 100 or more. Preferredresins have a melt flow ratio (MFR) of at least 60 with an MFR of atleast 100 being especially preferred and an MFR of at least 110 beingmost preferred. Processability of the inventive terpolymer film isgreatly enhanced by use of the higher MFR terpolymer resins. Also, goodfilm properties are exhibited with VLDPE terpolymer resins having meltflow ratios in excess of 60 with very good properties and processabilityevident in resins having an MFR greater than 110.

Suitable terpolymer resins employed in making the biaxially orientedfilms of the present invention may have a molecular weight distributionwhich ranges from narrow to broad. However, a VLDPE terpolymer resinhaving a broad molecular weight distribution is preferred. Resins with abroad molecular weight distribution greater than 10 as measured by ASTMD 3593 are preferred with a {overscore (M)}_(w)/{overscore (M)}_(n)greater than 12 especially preferred. It is believed that terpolymerresins having a broad molecular weight distribution have improvedprocessability via a tubular extrusion double bubble system such as thatdescribed in U.S. Pat. No. 3,456,044 (Pahlke). It is further believedthat biaxially stretched terpolymer very low density polyethylene filmsof the present invention such as C₂C₄C₆ VLDPE terpolymer which are madefrom broad molecular weight distribution resins have surprisingly goodproperties including puncture resistance relative to similar films madefrom narrow molecular weight distribution terpolymer resins. A biaxiallystretched film of the invention having a {overscore (M)}_(w)/{overscore(M)}_(n) of about 12.5 has been found to have excellent film propertiesincluding puncture resistance and shrinkage value.

Suitable terpolymer resins according to the invention will be made bypolymerization of three essential components. These three componentscomprise monomers (a), (b) and (c) wherein monomer (a) comprisesethylene, monomer (b) comprises a C₆-C₈ alpha-olefin, and monomer (c)comprises either 1-butene or 1-hexene. When monomer (c) comprises1-hexene, then monomer (b) must comprise a C₆-C₈ alpha olefin other than1-hexene. Terpolymers of the present invention will preferably have atleast one percent by weight of polymer units derived from monomer (b).

The C6-C8 alpha-olefin comprises any C6-C8 alpha-olefin having a singledouble bond such as 4-methyl-1-pentene, 1-hexene and 1-octene.Alpha-olefins containing more than one double bond are believed to formrubber like compositions whose rubber like properties are undesirable inthe present invention and are not employed as the necessary thirdcomponent of the terpolymer, although the alpha-olefins having two ormore double bonds may be added in small amounts as may many othermaterials as a minor fourth component or may be blended in with theterpolymer resins as a modifier. A preferred resin for forming a filmaccording to the present invention comprises a VLDPE terpolymer ofethylene, 1-butene and 1-hexene. Also, suitable terpolymer resins maycontain other components including processing aids, catalyst residues,and/or property enhancing additives. These suitable terpolymer resinsmay also be blended with one or more additional polymers or copolymerssuch as VLDPE, LDPE, HDPE, LLDPE, polypropylene, polyester, nylon, PVDC,EVA and ionomers.

Beneficially, the VLDPE terpolymer resins of the present invention willbe copolymerized by adding either 1-butene monomer or 1-hexene monomerand a C₆-C₈ alpha-olefin monomer such as 1-hexene or 1-octene monomer toan ethylene monomer under polymerization conditions such that theresultant very low density polyethylene terpolymer resin having adensity less than 0.915 g/cm³ will comprise at least 80 weight percentof its polymer units derived from ethylene and preferably at least 85weight percent polymer units derived from ethylene. As the amount of theethylene monomer component decreases, there is a tendency towards lesscrystalline materials which are increasingly elastomeric. Materialswhich are exceedingly elastomeric present one or more problems for foodpackage applications such as difficulty in controlling orientation to aset T.D. width, too soft to handle easily; weak puncture strength atelevated temperatures or in hot water; or excessive n-hexaneextractables which are undesirable in food packaging.

Films of the present invention may also be further distinguished fromundesirably rubber-like or elastomeric materials by melting point, Vicatsoftening point, and/or 1% secant modulus. Many synthetic rubbers lack acrystalline melting point. Terpolymer resins utilized in the presentinvention have a crystalline melting point which may be determined bydifferential scanning calorimetry (DSC) according to a method similar toASTM D-3418 using a 5° C. per minute heating rate and a DuPont 9000brand differential scanning calorimeter. Suitable resins may bedifferentiated from undesirably elastomeric materials by measurement ofthe crystalline melting point of suitable resins. This melting point isgenerally greater than about 110° C. Terpolymer resins having a meltingpoint greater than about 125° C. are disadvantageously and decreasingly(with higher temperatures) difficult to process into biaxially stretchedfood packaging films. VLDPE terpolymer resins useful in the presentinvention have a melting point which is preferably between 115° C. and125° C.

The Vicat softening point may also be used to further define the presentinventions. Films of the present invention utilize VLDPE terpolymerresins which generally have a Vicat softening point greater than about60° C. and preferably greater than 80° C. Materials having lower Vicatsoftening points are elastomeric rubber-like compositions which aredisadvantageously difficult to dimensionally control during biaxialstretching.

Suitable VLDPE terpolymer containing films according to the presentinvention will beneficially have a 1% secant modulus at least about10,000 p.s.i. (69 MPa). Films with lower values tend to be too soft forproper handling as food packaging films for use in e.g. poultry bags.Advantageously, food packaging films of the present invention will havea 1% secant modulus between about 10,000 to 40,000 p.s.i. (69-280 MPa);this range of softness provides a desirable degree of softness for easeof handling during both film manufacturing and food packagingoperations.

Suitable resins include those in which the ratio of C₆-C8 alpha-olefinto 1-butene or 1-hexene ranges from less than 1:1 to more than 3:1.Preferred terpolymer resins and films of the present invention havingabout a 3:1 ratio of polymer units derived from C₆-C₈ alpha-olefin topolymer units derived from 1-butene have been found to have a desireddegree of crystallinity as reflected in such properties as meltingpoint, 1% secant modulus and Vicat softening point as well as adesirable balance of film properties including high shrinkage values,dynamic puncture resistance and hot water puncture resistance. Anespecially preferred biaxially stretched, heat shrinkable film of thepresent invention will have at least 85 weight percent polymer unitsderived from ethylene and a ratio of polymer units derived from C₆-C₈alpha-olefin (1-hexene preferably) to polymer units derived from1-butene of about 3:1.

Advantageously, a preferred embodiment of the inventive heat shrinkablefilm will have a maximum extractable portion of 5.5 percent by weight ofpolymer in n-hexane at 50° C. for 2 hours as further described below.This 5.5 weight percent is the n-hexane extractactable limit for olefincopolymers of the type employed by the present invention for use inarticles that contact food except for articles used for packing orholding food during cooking. Beneficially, the maximum extractableportion as described above will be 2.6 percent in an especiallypreferred embodiment of the inventive film thereby qualifying the filmfor use in articles used in packing or holding food during cooking. Theabove maximum extractable limits correspond to current limits for aclass of resins intended for use in contact with food as set forth anddescribed by the U.S. Food & Drug Administration in 21 CFR 177.1520(which description is hereby incorporated in its entirety by reference).

Suitable ethylene, 1-hexene, 1-butene VLDPE terpolymer resins for makingthe biaxially stretched heat shrinkable films of the present inventionhave been produced by Union Carbide Corporation. It is believed that inview of the present disclosure, those of ordinary skill in the art ofmaking VLDPE resins may manufacture suitable terpolymer resins via knownprocesses without undue experimentation.

As generally recognized in the art, resin properties may be furthermodified by blending two or more resins together and it is contemplatedthat the terpolymer resins described above may be blended with otherresins such as other VLDPEs, LLDPE, LDPE, HDPE, ionomers, polypropyleneor EVA. These resins and others may be mixed by well known methods usingcommercially available tumblers, mixers or blenders. Also, if desired,well known additives such as processing aids, slip agents, antiblockingagents, pigments, etc., and mixtures thereof may be incorporated intothe film.

In a preferred process for making films of the present invention, theresins and any additives are introduced to an extruder (generally oneextruder per layer) where the resins are melt plastified by heating andthen transferred to an extrusion (or coextrusion) die for formation intoa tube. Extruder and die temperatures will generally depend upon theparticular resin or resin containing mixtures being processed andsuitable temperature ranges for commercially available resins aregenerally known in the art, or are provided in technical bulletins madeavailable by resin manufacturers. Processing temperatures may varydepending upon other process parameters chosen. For example, accordingto the present invention, in extrusion of the VLDPE terpolymers such asC₂C₄C₆ VLDPE terpolymers, barrel and die temperatures may range betweenabout 165° C. and 180° C. However, variations are expected which maydepend upon such factors as variation of terpolymer composition, use ofother resins e.g. by blending or in separate layers in a multilayerfilm, the manufacturing process used and particular equipment and otherprocess parameters utilized. Actual process parameters including processtemperatures are expected to be set by one skilled in the art withoutundue experimentation.

In a preferred extrusion double bubble process of the type described inU.S. Pat. No. 3,456,044 the primary tube leaving the die is inflated byadmission of air, cooled, collapsed, and then preferably oriented byreinflating to form a secondary bubble with reheating to the film'sorientation (draw) temperature range. Machine direction (M.D.)orientation is produced by pulling or drawing the film tube e.g. byutilizing a pair of rollers travelling at different speeds andtransverse direction (T.D.) orientation is obtained by radial bubbleexpansion. The oriented film is set by rapid cooling. Suitable machinedirection and transverse direction stretch ratios are from about 3:1 toabout 5:1 with a ratio of about 4:1 preferred.

Advantageously, thermoplastic biaxially stretched terpolymer films ofthe present invention exhibit one or more of the following properties:

(i) A dynamic puncture resistance greater than that for similarly madefilms comprising a copolymer of ethylene and a C₆-C₈ alpha-olefinwithout 1-butene.

(ii) A hot water puncture value of at least 20 seconds, preferably atleast 60 seconds, and most preferably at least 120 seconds.

(iii) A shrinkage value of at least about 15 percent in at least onedirection, (preferably at least 20 percent in the machine direction) anddesirably at least about 20 percent (preferably at least 25 percent andmost preferably at least 30 percent) in the transverse direction.

The following physical properties are used to describe the present filmand are measured in the described manner.

Dynamic Puncture Resistance

The dynamic puncture resistance procedure is used to compare films fortheir resistance to bone puncture. It measures the energy required topuncture a test sample with a sharp pyramidal metal point made tosimulate a sharp bone end. A Dynamic Ball Burst Tester, Model No. 13-8,available from Testing Machines, Inc., Amityville, Long Island, N.Y., isused, and a modified tip is installed on the tester probe arm for use inthis test procedure. The modified tip is constructed from a ⅜ inch (0.95cm) diameter conical tip having a configuration of a right circular conewith the angle between the cone axis and an element of the conicalsurface at the vertex being about 65°. Three equally spaced and abuttingplanar surfaces are machined to a smooth finish on the cone surface toform a pyramidal shaped point. At least six test specimens approximately4 inches (10 cm) square are prepared, a sample is placed in the sampleholder, and the pendulum is released. The puncture energy reading isrecorded. The test is repeated until at least 6 samples have beenevaluated. The results are calculated in cm-kg per mil of film thicknessand are averaged.

Hot Water Puncture

Hot water puncture values for monolayer films are obtained by performinga hot water puncture test as follows. Water is heated to 98±1° C. A ⅜inch (0.95 cm) diameter round wooden dowel is sharpened on one end to aconical point. This sharpened point has the configuration of a rightcircular cone, and the angle between the cone axis and an element of theconical surface at the vertex is about 60°. This sharp point is thenrounded to a spherical tip of about {fraction (1/16)} inch (0.16 cm)diameter. The wooden dowel is fastened to a seven inch (17.8 cm) longwooden block so that the rounded point projects 1½ inches (3.8 cm)beyond the end of the wooden block.

A specimen about 3 inches (7.6 cm) wide in the machine direction (MD)and about eighteen inches (45.7 cm) long is cut from the test samplematerial. One end of the specimen is placed on the end of the woodenblock opposite the pointed dowel. The specimen is wrapped around the endof the sharpened dowel and back to the wooden block on the oppositeside, where it is secured. The film thickness in the area of contactwith the sharpened dowel is measured in order to assure that the filmspecimen thickness is truly representative of the given test samplematerial.

The specimen and pointed dowel are quickly immersed five inches into thehot water and a timer is started. The timer is stopped when the woodendowel point punctures the film specimen. The test procedure is repeatedfive more times with new 3 inch (7.6 cm) wide MD specimens from thegiven test sample material. The time required for penetration isrecorded and then averaged for the six MD specimens. Resistance topuncture times of below 6-7 seconds are generally consideredunacceptable, while times of 20 seconds or more are good, 60 seconds ormore are very good and 120 seconds or more are excellent.

For multilayer films, the above procedure is followed except a similarlyshaped stainless steel metal probe having an angle of 37° is substitutedfor the wood dowel and the water is heated to 95 +/−1° C.

The multilayer hot water puncture test has been found to be more severethan the monolayer test and resistance to puncture of six seconds ormore is considered to be exceptionally good.

Shrinkage

The biaxially stretched films of the present invention are heatshrinkable. Biaxially stretched films are “heat shrinkable” as that termis used herein, if the film has an unrestrained shrinkage of at least 5percent in two directions.

Shrinkage values are obtained by measuring unrestrained shrink of thestretched film at 90° C. for five seconds. Four test specimens are cutfrom a given sample of the oriented film to be tested. The specimens arecut to 10 cm. in the machine direction by 10 cm. in the transversedirection. Each specimen is completely immersed for 5 seconds in a 90°C. water bath. The distance between the ends of the shrunken specimen ismeasured. The difference in the measured distance for the shrunkenspecimen and the original 10 cm. is multiplied by ten to obtain thepercent of shrinkage for the specimen. The shrinkage for the fourspecimens is averaged for the MD shrinkage values of the given filmsample, and the shrinkage for the four specimens is averaged for the TDshrinkage value.

Shrink Force

The shrink force of a film is that force or stress required to preventshrinkage of the film and was determined from film samples taken fromeach film. Four film samples were cut 1″ (2.54 cm) wide by 7″ (17.8 cm)long in the machine direction and 1″ (2.54 cm) wide by 7″ (17.8 cm) longin the transverse direction. The average thickness of the film sampleswas determined and recorded and a strip chart recorder was calibrated at0 gram and at 1,000 grams full scale load. Each film sample was thensecured between two clamps spaced 10 cm apart. One clamp is in a fixedposition and the other is connected to a strain gauge transducer. Thesecured film sample and clamps were then immersed in a silicone oil bathmaintained at a constant, elevated temperature for a period of fiveseconds. During this time, the force in grams at the elevatedtemperature was read from the strip chart and this reading was recorded.At the end of this time, the film sample was removed from the bath andallowed to cool to room temperature whereupon the force in grams at roomtemperature was also read from the strip chart and recorded. The shrinkforce for the film sample was then determined from the followingequation wherein the result is obtained in grams per mil of filmthickness (g/mil):${{Shrink}\quad {Force}\quad \left( {g/{mil}} \right)} = \frac{F}{T}$

wherein F is the force in grams and T is the average thickness of thefilm samples in mils.

The following are examples and comparative examples given to illustratethe present invention.

In all the following examples, unless otherwise indicated herein thefilm compositions were produced generally utilizing the apparatus andmethod described in U.S. Pat. No. 3,456,044 (Pahlke) (herebyincorporated by reference) which describes an extrusion type of doublebubble method and in further accordance with the detailed descriptionabove. In all the examples below, unless otherwise noted, the extrudedprimary tube was biaxially oriented following the Pahlke method andwound on a reel. Those skilled in the art of manufacturing biaxiallyoriented films know of different and various processes for suchmanufacture and the present inventive films include biaxially orientedor stretched films regardless of the method used for their production.All percentages are by weight unless indicated otherwise.

Unless otherwise noted, the physical properties reported in the examplesbelow were measured by either the test procedures described above ortests similar to the following methods.

Average Gauge: ASTM D-2103

Tensile Strength: ASTM D-882, method A

Secant Modulus: ASTM D-882, method A

Percent Elongation: ASTM D-882, method A

Molecular Weight Distribution: ASTM D-3593

Gloss: ASTM D-2457, 45° Angle

Haze: ASTM D-1003-52

Melt Index: ASTM D-1238, Condition E

Melt Flow Index: ASTM D-1238, Condition F

Melting Point: ASTM D-3418, DSC with 5° C./min. heating rate.

Vicat Softening Point: ASTM D-1525-82

All ASTM test methods noted herein are incorporated by reference intothis disclosure.

EXAMPLE 1

In example 1 a biaxially stretched, heat shrinkable film of the presentinvention was made and physical properties of the film tested. This filmwas made from an experimental resin supplied by Union CarbideCorporation comprising a VLDPE terpolymer of ethylene, 1-butene, and1-hexene having a density less than 0.915 g/cm³. The as-received resinhad a reported density of 0.905 g/cm³, a melt index of 0.22 g/10 min.,and a melt flow ratio (MFR) of 112. The melt flow ratio is the ratio ofthe melt flow index to the melt index. This most preferred VLDPEterpolymer resin had a broad molecular weight distribution which isreportedly 12.45. Molecular weight distribution may be measured by knownmethods e.g. size exclusion chromotography. This terpolymer resin isbelieved to have been made from at least 85 weight percent ethylenemonomer and an approximately 3:1 ratio of 1-hexene to 1-butene monomer.The Vicat softening point of this resin was reported at 82.5° C. Themelting point was measured at 121° C. by DSC.

This terpolymer resin was uniformly mixed with 4.4 weight percent of aprocessing aid comprising a 1.7% fluorocarbon elastomer in a LLDPE basesold by Quantum Chemical Corporation under the brand name NortechCM-1607, and 5 weight percent of a color concentrate and placed in ahopper attached to a standard single screw extruder equipped with astandard 1½ inch (3.81 cm) diameter annular die.

The resin mixture was fed from the hopper into the extruder and was heatplastified and extruded into a primary tube. The extruder barrel and dietemperature were set at about 350° F. (177° C.) and 370° F. (188° C.)respectively. The melt temperature was measured at about 160° C. at theextruder head and the melt pressure was about 5200 p.s.i. (36 MPa) asmeasured at the screw tip.

This primary tube was then biaxially stretched according to a doublebubble process and the resultant biaxially stretched film wound on areel. The machine direction (M.D.) orientation ratio was about 3.6:1 andthe transverse direction (T.D.) orientation ratio was about 4:1. Drawpoint temperature, bubble cooling rates and orientation ratio wereadjusted to maximize bubble stability.

The film produced in Example 1 processed well with no noticable gels ormelt fracture. The film had an average gauge of 2.50 mils (64 microns).The M.D./T.D. tensile strength at room temperature was measured at about9,300/10,100 p.s.i. (64/70 MPa), respectively, indicating good filmstrength relative to present commercial films such as those comprised ofethylene vinyl acetate (EVA) which generally have an M.D./T.D. tensilestrength of about 9,000/10,000 p.s.i. (62/69 MPa). The elongation atbreak at room temperature was measured to be about 225% in the machinedirection and 210% in the transverse direction. The 1% secant moduluswas measured at about 17,400 p.s.i. (120 MPa) in the machine directionand 19,400 p.s.i. (134 MPa) in the transverse direction indicating goodfilm handling characteristics e.g. in the gathering and closing an openend of a poultry bag. This film had excellent shrink characteristicswith M.D./T.D. shrinkage values of 27/35 percent. Also, there was goodhigh temperature (measured at 90° C.) M.D./T.D. shrink force of 105/165g/mil (41/65 kg/cm) and residual shrink force at room temperature of65/100 g/mil (26/39 kg/cm) generally equivalent to presentlycommercialized poultry bags made from EVA. The puncture resistanceproperties were also examined. The 2.2 cmkg/mil (87 cmkg/cm) (0.09cmkg/micron) dynamic puncture resistance was very good and the hot waterpuncture time at 98° C. of over 120 seconds for an average sample filmgauge of 2.69 mils (68.3 microns) was excellent for poultry bagapplications.

The physical properties of the above biaxially oriented, heat shrinkablethermoplastic flexible film were all very good for film packagingapplications with very desirable levels of shrinkability and punctureresistance. In particular, this film of the invention showed a uniqueand surprising combination of desirable puncture resistance propertieswith high shrinkage values. Individual film properties were as good orbetter than films used in commercially available poultry bags. Theunique combination of excellent high shrinkage values with excellent hotwater puncture resistance times and very good dynamic punctureresistance is previously unknown.

EXAMPLES 2-4

In examples 2-4 a series of monolayer films were made from differentvery low density polyethylene (VLDPE) resins. Several physicalproperties of these films were measured and are presented in Table 1.Examples 2-3 are comparative (not of the invention), while example 4 isa film according to the present invention. Example 1 as described aboveis included in the table as representative of an inventive film having abroad molecular weight distribution in contrast to the comparativeexamples (not of the invention) 2 and 3 and example 4 (a film of thepresent invention) all of which have a narrow molecular weightdistribution. Since some process conditions such as orientation ratiovaried between example 1 and the remaining examples in Table 1, thevalues obtained for example 1 should not be directly compared to thoseof examples 2-4, but nonetheless do indicate that a desirablecombination of properties may be obtained for the inventive films.

In comparative example 2 a VLDPE copolymer of ethylene and 1-butene(commercially available from Union Carbide Corporation (UCC) of Danbury,Conn. under the brand designation UCAR DFDA 1137 Natural 7) having areported density of 0.905 g/cc and a melt index of 1.0 was fed by hopperto a standard single screw extruder equipped with a standard 1½ inch(3.81 cm) diameter annular die. The resin was heat plastified andextruded into a primary tube. This primary tube was then biaxiallystretched according to a double bubble process and the resultantbiaxially stretched film wound on a reel as described above for example1.

In comparative example 3, a VLDPE copolymer of ethylene and 1-hexene(available as an experimental resin from UCC under the brand designationDEFD 1569) having a reported density of 0.910 g/cc and a melt index of1.0 was made into a biaxially oriented film by an extrusion type doublebubble process as described for examples 1-2.

In example 4 of the invention, a VLDPE terpolymer of ethylene, 1-butene,and 1-hexene (an experimental resin provided by Union CarbideCorporation) having a reported density of 0.904 g/cm³, a melting pointof about 122° C., and a 0.71 dg/min melt index was made into a biaxiallyoriented film by the process as described for examples 1-2. The meltflow index is reported at 24.4 dg/min. and melt flow ratio at 34.2 andthe Vicat softening point at 80.2° C. This resin of the invention aswell as the comparative examples 2-3 above has a narrow molecular weightdistribution (less than 10).

In all of the examples 2-4 the following process conditions wereutilized. The extruder barrel and die temperatures ranged from about350-375° F. (177-191° C.). The machine direction (M.D.) orientationratio was from about 4.1:1 to 4.3:1 and the transverse direction (T.D.)orientation ratio was from 4.0:1 to 4.2:1. Draw point temperature,bubble cooling rates and orientation ratios were adjusted to maximizebubble stability.

The average gauge of each film was measured by a method similar to ASTMD-2103 and various other physical properties measured by tests describedabove. These test results are reported in Table 1.

TABLE 1 Secant DYN. HOT Tensile Modulus AVG. PUNC. WATER SHRINK Strengthat 1% Elongation GAUGE cmkg/mil PUNC. at 90° C. × 10³ psi × 10³ psi atbreak VLDPE mil (cmkg/ sec./mil % (MPa) (MPa) % # Resin (micron) micron)/(micron) M.D./T.D. M.D./T.D. M.D./T.D. M.D./T.D. 1 C₂C₄C₆* 2.50 2.2120+/2.69 27/35 9.3/10.1 17.4/19.4 225/210 (64) (.089) /(68) (64/70)(120/134) 2 C₂C₄** 2.17 2.0 12/1.77 25/33 7.9/7.9 20.6/21.1 275/240 (55)(.079) /(45) (54/54) (142/145) 3 C₂C₆*** 1.92 2.0 120+/1.94 14/2310.7/10.8 31.6/34.9 210/195 (49) (.079) /(49) (74/74) (218/241) 4C₂C₄C₆**** 2.10 2.3 69/2.28 22/33 8.8/9.4 18.8/18.0 200/225 (53) (.091)/(58) (61/65) (130/124) *Experimental VLDPE terpolymer of ethylene,1-butene and 1-hexene having a density of 0.905 g/cc, a melt index 0.22,and a broad molecular weight distribution. **Commercially availableVLDPE copolymer of ethylene and 1-butene sold by Union CarbideCorporation of Danbury, Connecticut under the brand designation DFDA1137, Natural 7 and having a reported density of 0.906 g/cc and 1.0 meltindex. ***Experimental VLDPE copolymer of ethylene and 1-hexene sold byUnion Carbide Corporation of Danbury, Connecticut under the branddesignation DEFD 1569 and having a reported density of 0.905 g/cc and1.0 melt index. ****Experimental VLDPE terpolymer of ethylene, 1-buteneand 1-hexene having a density of 0.904 g/cc, a melt index of 0.71, and anarrow molecular weight distribution. +Ten samples were tested. Sixsamples with an average gauge of 1.80 mil had an average hot waterpuncture time of 84.6 seconds. Four samples having an average gauge of1.85 mil resisted puncture in excess of the maximum test time of 120seconds.

Referring to Table 1, the dynamic puncture resistance test was performedon six samples for each example and the averaged results reported.

Surprisingly, the dynamic puncture resistance is significantly improvedfor the inventive biaxially oriented, heat shrinkable flexible film ofexample 4, comprising an ethylene, 1-butene, 1-hexene very low densitypolyethylene relative to similarly produced films of the ethylene,1-butene VLDPE of comparative example 2 or the ethylene, 1-hexene VLDPEof comparative example 3. This higher puncture resistance for thebiaxially stretched C₂C₄C₆ terpolymer film would not be expected orpredicted from the results of comparative examples 2-3.

Shrinkable poultry bags need to have sufficient shrinkage to conform thefilm to the irregular shape of a bird during and after packaging. Eachfilm's unrestrained shrinkage at 90° C. for 5 seconds was measured inboth M.D. and T.D. directions and reported as a percentage of theoriginal dimensions. An average shrinkage percent for four samples isreported. Examples 1 and 4 of the present invention show very goodshrinkage values with over 20% shrinkage in the machine direction andover 30% shrinkage in the transverse direction.

Use of shrinkable films to package goods including red meat or poultrytypically entails passage of the packaged goods through a shrink tunnelor other means to apply elevated temperatures to the film to induceshrinkage.

Protuberances such as sharp bones or wing tips of birds may causepunctures during typical shrink procedures of packaging operations. Thehot water puncture resistance test measures the resistance to punctureunder conditions of elevated temperature. Typical shrink proceduresexpose products such as poultry or red meat to elevated temperatures forup to 6-8 seconds. Therefore, a minimum of 15-20 seconds resistance topuncture is desired at elevated temperature to provide a margin ofsafety to avoid the costs associated with defective packaging.Preferably, films suitable for packaging poultry will have an averagehot water puncture value of at least about 60 seconds and most preferredare films having a hot water puncture resistance of 120 seconds or more.As seen from the results in Table 1, examples 1 and 4 of the presentinvention and comparative example 3 meet or exceed the preferred sixtysecond time interval of puncture resistance at elevated temperature. Thecomparative ethylene, 1-butene film of example 2 has an undesirably lowvalue for hot water puncture resistance.

Although not reported in Table 1 since tests were not run on thecomparative films, the shrink force of the inventive film of example 4was measured at 90° C. to determine a film's ability to pull the wingsof a processed bird in close to the body of the bird. The required forceto do this is particularly high for turkeys. The residual force was alsomeasured at room temperature after the film cooled. This residual forceis very important to ensure a long lasting tight package. Relaxation ofthe film produces a poorer product appearance, increases the storagespace requirements for packaged birds and also increases the likelihoodthat the packaging film may be torn or otherwise damaged.

The inventive film of example 4 has a good M.D./T.D. shrink force of110/145 gm/mil (43/57 kg/cm) at initial shrinking temperatures (90° C.)and an acceptable residual force of 65/95 gm/mil (26/37 kg/cm) at roomtemperature for packaging items such as poultry.

Since the films of the present invention are made from new ethylene,1-butene, C₆-C₈ alpha-olefin VLDPE terpolymer resins, and biaxiallystretched film properties of these resins have not yet been taught inthe art, those of ordinary skill in the art would not be able to predictwith confidence the useful physical properties of the biaxiallystretched heat shrinkable terpolymer VLDPE films of the presentinvention. Especially unpredictable is the desirable combination of highshrinkage values and puncture resistance (particularly hot waterpuncture resistance) as demonstrated by the C₂C₄C₆ films of examples 1and 4. While methods have been previously disclosed in the art to makepolyethylene terpolymers, the biaxially stretched film properties ofethylene, 1-butene, C₆-C₈ alpha-olefin terpolymers such as ethylene,1-butene, 1-hexene VLDPE terpolymers have remained unexplored. One ofordinary skill in the art would not expect to be able to accuratelypredict the properties of these inventive terpolymers based uponknowledge of either two component copolymers of C₂C₄, or ethylene, C₆-C₈alpha-olefin, or be able to predict based upon other known terpolymerssuch as ethylene, propylene, and ethylidene norbornene (See EPpublication No. 120,503).

EXAMPLES 5-8

In examples 5-8, a series of monolayer films were made according to theprocess described above with respect to examples 1-4.

Example 5 is a comparative example (not of the invention) of a VLDPEfilm comprising a copolymer of ethylene and 1-octene (commerciallyavailable from The Dow Chemical Company of Midland, Mich. under thebrand designation Attane 4001) having a reported density of 0.912 g/ccand a 1.0 melt index. Examples 6-8 are of the invention.

Example 6 is a biaxially drawn film of the present invention comprisingan ethylene, 1-butene, 1-hexene VLDPE terpolymer having a density ofabout 0.901 g/cm³, a melt index of 0.24 dg/min., a melt flow index of17.1 dg/min, and a melt flow ratio (MFR) of about 71. The crystallinemelting point was measured at 118° C. using differential scanningcalorimetry, (DSC). This terpolymer composition had a broad molecularweight distribution with a {overscore (M)}_(w)/{overscore (M)}_(n) valueof 10.5 reported. {overscore (M)}_(w)/{overscore (M)}_(n) may bemeasured by size exclusion chromatography (ASTM 3593). This terpolymerresin was made with a reported 1-hexene: 1-butene monomer ratio of about3:1.

Example 7 is a biaxially drawn film of the present invention comprisingan ethylene, 1-butene, 1-hexene VLDPE terpolymer having a density ofabout 0.903 g/cm³, a melt index of about 0.25, a melt flow index ofabout 17.3 and a melt flow ratio of about 70. The crystalline meltingpoint was measured at 121° C. by DSC. This terpolymer also has a broadmolecular weight distribution (11.9) which may be measured by sizeexclusion chromatography. This terpolymer resin was made with a reported1-hexene: 1-butene monomer ratio of about 3:1.

Example 8 is a biaxially drawn film of the present invention comprisingan ethylene, 1-butene, 1-hexene VLDPE terpolymer having a density ofabout 0.903 g/cm³, a melt index of about 0.26 dg/min a melt flow indexof about 18.1 dg/min., and a melt flow ratio of about 69. Thecrystalline melting point was measured at 118° C. by DSC. Thisterpolymer has a broad molecular weight distribution and was made with areported 1-hexene: 1-butene monomer ratio of about 1:1. The ethylenemonomer content in forming the terpolymer resins of examples 7, 8 and 9was believed to exceed 80 weight percent.

Physical properties were measured as with examples 1-4 and the resultsreported in Table 2. Reported test values in Table 2 are not directlycomparable to those of Table 1 since orientation conditions variedslightly. In particular, for examples 5-8 the machine direction (M.D.)orientation ratio (draw ratio) was from 4.6:1 to 4.8:1 and thetransverse direction (T.D.) orientation ratio was from 3.7:1 to 3.8:1.

Referring to Table 2, all films made from terpolymer resins according tothe present invention (examples 6-8) are shown to have very good dynamicpuncture and hot water puncture resistance relative to commercializedfilms used in poultry bags. Typical values for commercial films are 3-6cmkg for dynamic puncture resistance and 20-120 seconds for hot waterpuncture resistance for a single layer film of 2.25-2.5 mils (57-64microns) thickness.

Example 5 is a comparative example of a VLDPE copolymer of ethylene and1-octene which is a highly regarded resin currently used in commercialheat shrinkable packaging films. Octene monomer is generally moreexpensive than butene or hexene monomer and very low densitypolyethylene made from octene monomer generally has better punctureresistance properties than 1-butene based VLDPE. Also, known twocomponent ethylene, 1-octene VLDPEs have lower maximum shrinkage valuescompared to either 1-butene based two component VLDPE or to the presentinvention. Higher shrinkage values contribute to improved packageappearance.

As seen from Table 2, the inventive biaxially stretched, heat shrinkableVLDPE terpolymers are superior to 1-octene based VLDPE in shrinkagevalues and machine direction shrink force. The inventive films and filmof the comparative example all have excellent hot water puncture timesand very good dynamic puncture values. In particular, example 8 of theinvention shows exceptionally good shrink which is greatly superior tothe shrinkage values for comparative example 5 and at the same time hasvery good dynamic puncture resistance values and excellent hot waterpuncture resistance times.

The measured values of tensile strength, secant modulus and elongationat break indicate that the biaxially oriented, heat shrinkable flexiblefilms made from VLDPE terpolymer according to the present invention arefilms having sufficient strength and flexibility for a variety ofpackaging applications including food packaging. Relative to the octenebased two component VLDPE film of comparative example 6, the novelterpolymer VLDPE films all are easier to handle as indicated by theirsubstantially lower secant modulus values while maintainingapproximately equivalent tensile strength. In food packagingapplications, bags made from softer films (lower secant modulus) areeasier to gather and close by clipping means.

However, films having a 1% secant modulus below 10,000 p.s.i. (69 MPa)tend to be too soft for ease of handling. Ideally, films will be softenough to be easily manipulated in the packaging process e.g. forgathering and closing an open bag end, yet these films will also be hardenough to have sufficient body to be easily manipulated and not be sosoft as to be limp or clingy. Films with a 1% secant modulus betweenabout 10,000 to 40,000 p.s.i. (69-280 MPa) provide a desirable degree ofsoftness which facilitates handling.

Table 2 further demonstrates that biaxially stretched, heat shrinkableC₂C₄C₆ terpolymer films of the present invention provide films havingphysical properties which are comparable to VLDPE film made from ahigher monomer viz 1-octene. Moreover, it is believed without wishing tobe bound by that belief that biaxially stretched, heat shrinkable filmsmade from terpolymer resins of ethylene, 1-butene, and a C₆-C₈alpha-olefin such as ethylene, 1-butene 1-hexene VLDPE terpolymershaving a low melt index have improved film properties for packagingapplications relative to similar resins having a relatively high meltindex of 1.0 dg/min or higher.

The low melt indices of the preferred terpolymers (including theespecially preferred terpolymer of example 1) necessitate that theseresins have a higher melt flow index for processability. The melt flowratio (MFR) which is a ratio of the melt flow index to the melt index isa measure of the processability of the suitable and preferred terpolymerresins used in the films of the present invention. Advantageously, forease of processability during film manufacture the melt flow ratio willbe greater than 65 and preferably greater than 100. It is furtherbelieved that all of the terpolymer resins in examples 6, 7 and 8 wereformed with an ethylene content of at least 85 weight percent and with a3:1 ratio of 1-hexene monomer to 1-butene monomer except for example 8which is believed to have a 1:1 ratio of 1-hexene to 1-butene.

TABLE 2 Secant DYN. HOT SHRINK FORCE Tensile Modulus AVG. PUNC. WATERSHRINK at 90° C. at RT Strength at 1% Elongation GAUGE cmkg/mil PUNC. at90° C. gm/mil gm/mil ×10³ psi × 10³ psi at break VLDPE mil (cmkg/sec./mil % (Kg/cm) (Kg/cm) (MPa) (MPa) % # Resin (micron) micron)/(micron) M.D./T.D. M.D./T.D. M.D./T.D. M.D./T.D. M.D./T.D. M.D./T.D. 5C₂C₈* 2.49 2.6 120+/2.58 18/27 115/185  60/130 12.5/12.1 22.8/21.4205/235 (63) (.10)  /(66) (45/73) (24/51) (86/83) (157/148) 6 C₂C₄C₆**2.26 2.3 120+/2.09 45/47 155/150 75/75 12.3/11.4 12.8/13.6 190/210 (57 (.091) /(53) (61/59) (30/30) (85/79) (88/94) 7 C₂C₄C₆*** 2.34 2.3120+/1.93 40/43 160/150 75/75 12.8/11.1 14.0/15.2 200/200 (59) (.091)/(49) (63/59) (30/30) (88/77) (97/105) 8 C₂C₄C₆**** 2.27 2.5 120+/1.9642/45 150/145 70/80 10.8/10.1 14.8/15.1 195/215 (58) (.098) /(50)(59/57) (28/31) (74/70) (102/103) *Commercially available VLDPEcopolymer of ethylene and 1-octene sold by the Dow Chemical Company ofMidland, Michigan under the brand designation Attane 4001 and having areported density of 0.912 g/cc and 1.0 melt index. **Experimental C₂C₄C₆VLDPE terpolymer provided by Union Carbide Corporation of Danbury,Connecticut having a reported density of 0.901 g/cc, and a 0.24 dg/min.melt index. ***Experimental C₂C₄C₆ VLDPE terpolymer provided by UnionCarbide Corporation of Danbury, Connecticut having a reported density of0.903 g/cc and 0.25 dg/min melt index. ****Experimental VLDPE terpolymerof ethylene, 1-butene and 1-hexene provided by Union Carbide Corporationof Danbury, Connecticut having a density of 0.903 g/cc and a melt indexof 0.26.

EXAMPLES 9-12

The suitability of an ethylene, 1-butene, and a C₆-C₈ alpha-olefin VLDPEterpolymer in at least one layer of a biaxially stretched, heatshrinkable multilayer film for packaging fresh red meat or processedmeat was examined.

In examples 9-12, four multilayer films were coextruded and biaxiallyoriented according to a coextrusion type of double bubble process suchas that described in U.S. Pat. No. 3,456,044 (Pahlke). This process wassimilar to that described above for examples 1-4 except that oneextruder was used for each layer and the heat plastified resins fromeach extruder were introduced to a coextrusion die from which resinswere coextruded in a first outer:core:second outer layer ratio of about5:2:3.

Examples 9-12 are three layered films. However, multilayer films of 2 or4 or more layers are contemplated by the present invention. Theinventive multilayer films may include tie or adhesive layers as well aslayers to add or modify various properties of the desired film such asheat sealability, toughness, abrasion resistance, puncture resistance,optical properties, gas or water barrier properties, and printability.These layers may be formed by any suitable method including coextrusion,extrusion coating, and lamination.

Such additional film layers may comprise such polymers as linear lowdensity polyethylene, very low density polyethylene, low densitypolyethylene, high density polyethylene, ionomer, ethylene-vinyl acetatecopolymer, nylon or mixtures thereof.

In examples 9-12 the coextruded film was oriented as for examples 1-4except as noted below. The extruder barrel temperatures for the corelayer ranged from 280 to 290° F. (138-143° C.) and for the second outerlayer ranged from about 325 to 335° F. (163-168° C.) and the first outerlayer ranged from about 300 to 320° F. (149-160° C.). The coextrusiondie temperature profile was set at about 275° F. (135° C.) to about 310°F. (154°C.). The M.D. orientation ratio was 4.0:1 to 4.5:1 and the T.D.orientation ratio was 3.9:1 to 4.7:1 for all films.

In all the examples 9-12, the extruded primary tube was wound on a reeland subsequently biaxially oriented following the Pahlke method. Aninterval of about one day occured between extrusion of the primary tubeand biaxial orientation due to equipment availability. It is believedthat this delay promoted crystallization in the primary tube therebyreducing shrinkage of the film. It is further contemplated that use of aprocess which is continuous from primary extrusion through biaxialorientation will provide increased shrink percentages for films whichare otherwise similarly made. Those skilled in the art of manufacturingbiaxially oriented films know of different and various processes forsuch manufacture and the present inventive films include biaxiallyoriented or stretched films regardless of the method used for theirproduction.

The average gauge and other physical properties were measured and arereported in Table 3. For all the examples 9-12, the core layer compriseda 3:1 blend of commercially available vinylidene chloride-methylacrylatecopolymer and vinylidene chloride-vinyl chloride copolymer and the outerlayer comprised a linear low density polyethylene (LLDPE). The corelayer and second outer layer resins used in examples 10-12 wereidentical to the resins used in example 9. All the VLDPEs used inexamples 9-12 were mixed with 25% by weight of the total resin mixturefor that layer of a commercially available ethylene vinyl acetatecopolymer (10% vinyl acetate), and 4.4 weight percent of a 1.7%fluorocarbon elastomer in a LLDPE base processing aid such as that soldby Quantum Chemical Corp. under the brand name Nortech CM-1607.

In another aspect of the invention, one or more layers having gasbarrier properties may be incorporated into multilayer film as either anintermediate or surface layer or both. For example, ethylene vinylalcohol copolymer (EVOH), vinylidene chloride methacrylate copolymer,nylons such as nylon 6, or amorphous nylon, vinylidene chloride-vinylchloride copolymer, acrylonitriles or other materials having oxygenbarrier properties may be used in one or more layers such as the corelayer.

Example 9 is a comparative example not of the invention and comprised afirst outer layer having an experimental resin (provided by UnionCarbide Corp. of Danbury, Conn.) having a 1-butene VLDPE with a reporteddensity of 0.905 g/cm³ and a melt index of 0.25 dg/min. This resin has areportedly broad molecular weight distribution.

Example 10 is a comparative example (not of the invention). Thecoextruded multilayer film of example 10 was similar in composition toexample 9 except that the first outer layer was comprised of anexperimental ethylene, 1-hexene VLDPE produced by Union CarbideCorporation of Danbury, Conn. having a reported density of 0.905 g/cm³and a melt index of 0.25 dg/min. This resin reportedly has a broadmolecular weight distribution. This resin was blended with a processingaid and color concentrate as described for example 9.

Example 11 is a comparative example (not of the invention). Thecoextruded multilayer film of example 11 was similar in composition toexample 9 except that the VLDPE of the first outer layer was comprisedof a 1-octene VLDPE sold by Dow Chemical Co. of Midland, Mich. under thebrand name Attane 4001. This is the same resin previously described inexample 5. This resin reportedly has a narrow molecular weightdistribution.

Example 12 is an example of the present invention. The coextrudedmultilayer film of example 12 was similar in composition to that ofexample 9 except that the VLDPE of the first outer layer comprised anethylene, 1-butene, 1-hexene VLDPE terpolymer according to the presentinvention. This VLDPE terpolymer resin used in example 12 is the same asthat discussed in example 1 above and has a reported density of 0.905g/cm³, a Vicat softening point of 82.5° C., a melt index of 0.22 dg/min,a melt flow index of 24.7 dg/min, a MFR of about 112 and a broadmolecular weight distribution of 12.45. The melting point of thisterpolymer resin was measured to be 121° C. by differential scanningcalorimetry. It is believed that this terpolymer resin was made with atleast 85% weight percent ethylene and a 3:1 ratio of 1-hexene to1-butene.

Example 12 demonstrates that a coextruded multilayer film having atleast one layer comprising a terpolymer VLDPE of ethylene, 1-butene anda C⁶-C₈ alpha-olefin may be made having useful properties for shrinkpackaging of, for example, fresh red meat. In particular, multilayerfilms according to the present invention may be usefully employed topackage fresh red meat including primal and subprimal cuts as well ascheese or other food products.

It is contemplated that additional resins may be added to the VLDPEterpolymer in accordance with the present invention in amounts of up to50 weight percent or more. Indeed it is believed that minor amounts(less than 50 weight percent) or small amounts (less than 10 weightpercent) of the disclosed VLDPE terpolymer may be usefully employed tomodify or blend with other resins such as LLDPE, VLDPE, LDPE, HDPE,ionomer, EVA or polypropylene in forming useful biaxially oriented, heatshrinkable films according to the present invention. The VLDPEterpolymer of the present invention may be utilized in one or moreintermediate or surface layers or combination thereof in a multilayerfilm.

Referring to Table 3, several properties of the inventive film ofexample 12 are compared to those for a film made with a commercial VLDPEresin such as that in example 11 having a narrow molecular weightdistribution that has gained wide acceptance in packaging films, andalso to two experimental VLDPE resins both having a broad molecularweight distribution (examples 9 and 10). It is seen that the inventivebiaxially oriented multilayer film of example 12 has very good dynamicpuncture resistance and equivalent shrinkage and acceptable hot waterpuncture times relative to the films of the comparative examples 9-11.Also, the glossiness of the inventive multilayer film of example 13compares favorably with the multilayer films of comparative examples9-11.

EXAMPLES 13-16

The multilayer films of examples 9-12 were irradiated after orientationwith 3.5 Mrad by electron beam according to methods well known in theart. The irradiated examples of 13-16 corresponds to unirradiatedexamples 9-12.

Physical properties of the irradiated multilayer films were tested andare reported in Table 3. The novel film of the present invention(example 16) showed very good to excellent puncture properties withdynamic puncture resistance as high or higher than butene, hexene, oroctene-based two component VLDPE copolymer films of examples 13-15. Thedynamic puncture resistance value of the inventive film remained verygood following irradiation and the hot water puncture resistance timesof all films were improved by irradiation to excellent values. Hot waterpuncture for all multilayer films tested herein were conducted using ametal probe at 95° C. as described earlier. Use of a metal probe is amore strenuous test of puncture resistance. Irradiation usually reducesthe shrinkage values, however the shrinkage values for the inventivefilm of example 16 are acceptable and compare favorably to the butene,hexene and octene comparative examples 13-15. The shrinkage values forthe octene based VLDPE film of comparative example 15 are undesirablylow for food packaging applications.

Two additional tests were conducted on the irradiated films namelyshrink force and impulse seal range. The shrink force was measured atelevated temperature and also the residual shrink force was examined.Both shrink force values of the inventive film were generally superiorto those of comparison films. The impulse seal range for comparativeexample 14 was undesirably narrow. The inventive film had a good rangecomparable to comparative example 16 which utilized an octene basedVLDPE widely used in biaxially oriented, heat shrinkable and sealablefood packaging films.

Examples 13-16 demonstrate that properties such as high temperaturepuncture resistance of the inventive multilayer films, may be improvedby irradiation. It is further believed that the same may be improved forinventive single layer films by either irradiation e.g. by an electronbeam and/or chemical crosslinking according to known methods.Preferably, the entire film is irradiated after orientation.Alternatively, one or more single layers may be oriented and irradiatedand optionally formed into a multilayer film by lamination processeswith other irradiated or nonirradiated layers. A suitable irradiationdosage is irradiation up to 10 Mrad with irradiation from 1 to 5 Mradpreferred. Known irradiation procedures may be utilized. Variousprocedures are described in U.S. Pat. No. 4,044,187. Irradiation isutilized to improve heat sealing characteristics. Excess irradiation maycause deleterious film discoloration and/or a reduction in shrinkagevalues.

In another aspect of this invention, bags suitable for theshrink-packaging of food articles such as poultry, primal meat cuts, andprocessed meats are provided from the aforedescribed films. The bags areproduced from the monolayer and multilayer films of this invention byheat sealing. For instance, if the films of this invention are producedin the form of tubular film, bags can be produced therefrom by heatsealing one end of a length of the tubular film or by sealing both endsof the tube end, then slitting one edge to form the bag mouth. If thefilms of this invention are made in the form of flat sheets, bags can beformed therefrom by heat sealing three edges of two superimposed sheetsof film. When carrying out the heat sealing operation, in one embodimentof the invention, the surfaces which are heat sealed to each other toform seams are the first outer layers of the multilayer films of theinvention. Thus, for example, when forming a bag by heat sealing oneedge of a length of a tubular film, the inner surface of the tube, i.e.,the surface which will be heat sealed to itself, will be the first outerlayer of the film. In other embodiments of the invention, the VLDPEterpolymer containing layer may be a core layer, a second outer layer oran intermediate layer. Also, the above noted VLDPE terpolymer of monomer(a) comprising ethylene, monomer (b) comprising a C₆-C₈ alpha-olefin andmonomer (c) comprising 1-butene or 1-hexene, may be incorporated in one,two, three or more layers of a multilayer film.

The voltage range for impulse sealing of film was also examined todetermine the acceptable range for producing a seal of sufficientstrength and integrity. In this test two four inch wide (T.D. direction)samples are cut from a tubular film. An impulse sealer equipped withcoolant flow controls for impulse time, cooling time, seal bar, coolantflow and pressure was set at the following conditions:

0.5 seconds impulse time (upper ribbon only)

2.2 seconds cooling time

50 p.s.i. (345 kpa)jaw pressure

0.3 gallon per minute (1 liter per minute) cooling water flow

One of the samples is folded in half for use in determining a minimumsealing voltage. This folding simulates folding which may inadvertentlyoccur during conventional bag sealing operations. The folded samplewhich now has four layers is placed into the sealer and by trial anderror the minimum voltage to seal the bottom two layers to each other isdetermined.

The maximum voltage is then determined for the two layer sample byplacing it in the sealer and then activating the seal bar. The filmsample is manually pulled with about 0.5 pounds of force to induce sealburn-through. The maximum voltage which does not cause burn-through orsignificant distortion of the seal is determined. The minimum andmaximum seal voltage are reported in Table 3. A broad impulse seal rangeas measured by the maximum and minimum voltage range is desirable foravoiding weak or noncontinuous seals and seal distortion andburn-through. A broad range allows for greater film variability andreduces the potential for operator error and further allows for greaterflexibility in sealing operations. Referring to the examples, theinventive VLDPE terpolymer containing film has a suitable impulse sealrange which is equivalent to that of comparative example 15 whichutilized a commercially accepted octene based VLDPE resin. The impulseseal range of the butene based VLDPE containing comparative film ofexample 13 was also acceptable whereas the range for the hexene basedVLDPE containing comparative film was undesirably narrow.

TABLE 3 MULTILAYER FILM COMPONENTS SHRINK FORCE IMPULSE AVG. DYN. WATERSKRINK at 90° C. at RT First Second SEAL GAUGE PUNC. PUNC. at 90° C.gm/mil gm/mil GLOSS Outer Core Outer RANGE mil cmKg/mil sec./mil (%)(Kg/cm) (Kg/cm) at 45° # Layer⁺⁺ Layer Layer (VOLTS) (micron) (/micron)/(micron) M.D./T.D. M.D./T.D. M.D./T.D. Angle  9 1-butene PVDC LLDPE —1.77 2.4 120+/1.77 17/21 — — 83 VLDPE* blend⁺⁺⁺ (45) (.095) /(45) 101-hexene PVDC LLDPE — 2.12 2.3 120+/2.09 16/20 — — 80 VLDPE** blend⁺⁺⁺(54) (.091) /(53) 11 1-octene PVDC LLDPE — 1.87 1.7 11/1.82 14/22 — — 78VLDPE*** blend⁺⁺⁺ (47) (.067) /(46) 12 C₂C₄C₆ PVDC LLDPE — 1.96 2.628/1.95 18/22 — — 80 VLDPE**** blend⁺⁺⁺ (50) (.10) /(50) 13⁺ 1-butenePVDC LLDPE 30-48 ⁺⁺⁺⁺ 2.5 120+/1.90 17/20 100/85 65/70 — VLDPE* blend⁺⁺⁺(.098) /(48) (39/33) (26/28) 14⁺ 1-hexene PVDC LLDPE 36-47 ⁺⁺⁺⁺ 2.1120+/2.37 16/18 120/90 75/70 — VLDPE** blend⁺⁺⁺ (.083) /(60) (47/35)(30/28) 15⁺ 1-octene PVDC LLDPE 31-50 ⁺⁺⁺⁺ 1.9 120+/1.90 14/17 70/9560/70 — VLDPE*** blend⁺⁺⁺ (.075) /(48) (28/37) (24/28) 16⁺ C₂C₄C₆ PVDCLLDPE 32-50 ⁺⁺⁺⁺ 2.6 120+/2.16 14/21 115/110 75/85 — VLDPE**** blend⁺⁺⁺(.10) /(55) (45/43) (30/33) RT — Room Temperature *An experimental VLDPEcopolymer of ethylene and 1-butene provided by Union Carbide Corp. ofDanbury, Connecticut. **An experimental VLDPE copolymer of ethylene and1-hexene provided by Union Carbide Corp. of Danbury, Connecticut. ***Acommercially available VLDPE copolymer of ethylene and 1-octene sold byThe Dow Chemical Company under the brand name Attane 4001. ****Anexperimental ethylene, 1-butene, 1-hexene very low density polyethyleneterpolymer provided by Union Carbide Corporation of Danbury,Connecticut. ⁺These films were irradiated at a dosage of 3.5 Mrad.⁺⁺Each inner layer component was blended with 25% by weight of anethylene-vinyl acetate copolymer (EVA). ⁺⁺⁺A 3:1 blend of a commerciallyavailable vinylidene chloride-methylacrylate copolymer and vinylidenechloride-vinyl chloride copolymer. ⁺⁺⁺⁺Not determined. However, theaverage gauge is expected to be similar to the values for the similarbut unirradiated films of examples 9-12 since samples of the films madefor examples 9-12 were irradiated to arrive at examples 13-16.

Further modifications of the invention disclosed will be apparent tothose skilled in the art and all such modifications are deemed to bewithin the scope of the invention as defined by the following claims.

What is claimed is:
 1. A biaxially stretched, heat shrinkable filmcomprising a terpolymer of monomers (a), (b) and (c), wherein monomer(a) comprises ethylene, monomer (b) comprises 1-hexene, and monomer (c)comprises 1-butene, wherein said terpolymer has a density less than0.915 g/cm³ and said terpolymer has a melt index of about 0.25 g/10 min.2. A biaxially stretched, heat shrinkable film comprising a terpolymerof monomers (a), (b) and (c), wherein monomer (a) comprises ethylene,monomer (b) comprises 1-hexene, and monomer (c) comprises 1-butene,wherein said terpolymer has a density less than 0.915 g/cm³ and saidterpolymer has a melt flow ratio of at least
 65. 3. A film, as definedin claim 2, wherein said terpolymer has a melt index of less than about1.0 dg/min.
 4. A biaxially stretched, heat shrinkable film comprising aterpolymer of monomers (a), (b) and (c), wherein monomer (a) comprisesethylene, monomer (b) comprises 1-hexene and monomer (c) comprises1-butene, wherein said terpolymer has a density less than 0.915 g/cm³and said terpolymer has a molecular weight distribution of at least 10.5. A film, as defined in claim 4, wherein said terpolymer has amolecular weight distribution of at least
 12. 6. A film, as defined inclaim 4, wherein said film has a density between about 0.900 and 0.910g/cm³; a melt index of less than about 1.0 dg/min, and at least 85percent polymer units derived from ethylene.
 7. A film, as defined inclaim 6, wherein said 1-hexene component of said terpolymer is presentin a weight ratio of about 3:1 of 1-hexene relative to 1-butene.
 8. Afilm, as defined in claim 7, wherein said film has a shrinkage value ofat least 20 percent in the machine direction and at least 30 percent inthe transverse direction.